Mass spectrometry comprises a broad range of instruments and methodologies used to elucidate the structural and chemical properties of molecules, to identify the atoms and molecules that compose samples of physical and biological matter, and to quantify the atoms and molecules identified in such samples. Mass spectrometers can detect minute quantities of pure substances (on the order of or less than 10−15 g) and, as a consequence, can identify compounds at very low concentrations (on the order of or less than one part in 1012) in chemically complex mixtures. The power of this analytical technique is evidenced by the fact that mass spectrometry has become a necessary adjunct to research in every division of natural and biological science and provides valuable information to a wide range of technologically based professions (e.g., medicine, law enforcement, process control engineering, chemical manufacturing, pharmacy, biotechnology, food processing and testing, and environmental engineering). In these applications, mass spectrometry is used to identify structures of biomolecules (such as carbohydrates, nucleic acids and steroids); to sequence biopolymers (such as proteins and oligosaccharidcs); to diagnose disease; to determine how drugs are used by the body; to perform forensic analyses (e.g., determine the presence and quantities of drugs of abuse); to assay environmental samples for pollutants; to determine the age and origins of geochemical and archaeological specimens; to identify and quantify components of complex organic mixtures; and to perform elemental analyses of inorganic materials (e.g., minerals, metal alloys, and semiconductors).
A mass spectrometer typically comprises an ion source, a mass analyzer, a detector, and a data handling system. The ion source's task is to convert atoms and molecules into gas-phase ions so they can be transported through the instrument under the action of electric and magnetic forces. Ions are transferred from the ion source into the mass analyzer where they are dispersed according to their mass-to-charge (m/z) ratios or a related mechanical property, such as velocity, momentum, or energy. At present, the most widely used types of mass analyzers are magnetic sectors, quadrupole mass filters, quadrupole ion-traps, time-of-flight tubes, and Fourier transform ion cyclotron resonance (FT ICR) cells. After the mass analyzer separates the ions, they interact with the detector to generate current or voltage signals, either of which has a magnitude proportional to the number of ions that produced it. These electrical signals, whatever their form; can be continuously processed, stored, and displayed on a monitor over the course of an analysis by a computerized data system; at the end of the analysis, they can be printed out on paper as a graph of signal intensity versus m/z, i.e. as a mass spectrum. In principle, the pattern of ion-signals that appears in the mass spectrum of a pure molecular substance constitutes a unique fingerprint from which the molecule's mass and various features of its structure can be deduced.
Mass spectrometry can be performed on a molecular sample in multiple, tandem stages to probe incisively into the complexities of molecular structure and to markedly increase specificity and sensitivity in analyses of complex mixtures of molecules. If the sample is a pure compound, a product-ion tandem analysis (FIG. 1A) can provide much additional information about the analyte's structure. If the sample is a mixture of compounds, a precursor-ion tandem analysis (FIG. 1B) can be used to uniquely identify a number of the mixture's molecular components; in this latter application, the procedure substantially increases signal-to-background ratios (and, thus, reduces limits of detection) by eliminating interferences from compounds of noninterest.
A tandem mass spectrometric unit, commonly designated as MS/MS or MS2, comprises two transmission mass analyzers (e.g., magnetic sectors, quadrupole mass filters, time-of-flight tubes, or a hybrid combination of such analyzers) arranged to perform spatially separated mass analyses in sequence (FIG. 1C), a single three-dimensional (3D) trapping mass analyzer (e.g., quadrupole ion-trap or FT ICR cell) that can perform two or more temporally separated mass analyses in sequence (FIG. 1D), or a hybrid arrangement of both transmitting and 3D trapping analyzers. In the first phase of a product-ion tandem mass analysis (precursor selection), a packet of ions of a particular m/z value, which are called precursor ions or precursors, is selected from among all the ions of various masses formed in the source as shown in FIG. 1A. In a transmission instrument, the first analyzer performs this operation, and in a 3D trapping instrument, the analyzer itself performs it. In the first phase of a precursor-ion tandem mass analysis (precursor scan), the precursors are spatially resolved from one another by the first analyzer of a transmission instrument. A precursor-ion analysis cannot be performed on a 3D trapping instrument. In the second phase (fragmentation), the precursor ions are induced to dissociate by a physicochemical process (FIGS. 1A and 1B). In a transmission instrument, this induced fragmentation takes place in a cell located between the two analyzers (FIG. 1B), and in a 3D trapping instrument, it takes place in the mass analyzer itself (FIG. 1C). In the third phase of a product-ion analysis (product-ion selection), the ionic fragments resulting from the dissociation process are resolved into a product-ion mass spectrum (FIG. 1A). In a transmission instrument, the second analyzer performs this operation, and in a 3D trapping instrument, the analyzer itself performs it. In the third phase of a precursor-ion analysis (FIG. 1B), only a certain ionic fragment from the dissociation of a particular precursor is transmitted by the second analyzer of the transmission instrument on which the analysis is being performed. The MS2 sequence can be extended to an MS3 sequence by using the second mass analyzer in a transmission instrument or the second round of mass dispersion in a 3D trapping instrument to select a packet of particular product ions from the preceding fragmentation stage as the precursors for a second level of fragmentation and product-ion analysis. This pattern can be repeated for yet higher orders of tandem analysis (MSn) so long as the number of product ions from a given stage of fragmentation is sufficient to produce an interpretable mass spectrum in the subsequent stage of mass analysis.
A gaseous molecular ion can be decomposed into fragments if its internal energy can be raised sufficiently during an interaction with a physical or chemical agent. The physicochemical processes most commonly used in MS/MS to fragment precursor ions are photon-induced dissociation (PID), low-energy collision-induced dissociation (CID), high-energy CID, electron impact excitation of ions from organic (EIEIO), electron transfer dissociation (ETD), electron capture dissociation (ECD), and electron detachment dissociation (EDD). In current practice, PID, low-energy CID, and high-energy CID are used universally to analyze all types of molecules whereas ETD, ECD, and EDD are used almost exclusively in the analysis of peptides and proteins. ECD, EDD, and ETD exhibit little selectivity for particular amino acids (proline and amino acids associated with disulphide bonds are exceptions); in addition, all three preserve labile post-translational modifications (PTMs), e.g., phosphorylation, o-glycosylation, and n-glycosylation. Consequently, these three dissociation processes are particularly suitable for analyzing peptides having as many as 20-25 amino acids and for determining the sites and nature of PTMs.
Each disassociation process induces fragmentation by forcing transitions in the precursor ions from bonding energy states to antibonding energy states. In PID, infrared photons induce nonpredetermined bonds to break by exciting various rotational and vibrational states, and ultraviolet photons of a specific wavelength induce predetermined bonds to break by exciting particular electronic states. PID requires an arrangement by which the precursor ions can be irradiated with an intense beam of photons; using a laser as the light source and an arrangement of common optical components, PID can (with little difficulty) be made to take place in any type of transmission dissociation cell or 3D analyzer. In CID, gas-phase collisions between precursors and inert atoms (like helium) or molecules (like nitrogen) induce nonpredetermined bonds to break by exiting various rotational, vibrational and electronic states. Low-energy CID and high-energy CID alike require that the precursor ions be intimately confined with the collision gas at a relatively high pressure. In current practice, low-energy CID is carried out most efficiently in 2D RF-multipole (e.g., quadrupole, hexapole, or octapole) ion-guides or 3D RF-trapping analyzers (e.g. quadrupole ion-traps or FT ICR cells), and high-energy CID is carried out in electric and magnetic field-free transmission cells designed to differentially maintain the collision gas at a relatively high pressure.
In ETD, exothermic single-electron-transfers from anions (which function both as bases and one-electron reducing agents) to multiply protonated peptidic precursors induce cleavage almost exclusively of the peptides' N-Cα (amine) backbone-bonds by exciting electronic states associated with the latter. ETD requires that the cationic precursors be intimately confined in space and time with anionic reagent molecules; this condition can be achieved in the 2D RF field of a linear multipole ion guide by applying a secondary RF-voltage to the multipole's end lenses. In ECD, exothermic single-electron-captures of free, low-energy (on the order of 1 eV for “normal” ECD and 20 eV for “hot” ECD) electrons by multiply protonated (cationic) peptidic precursors induce the peptides' N-Cα backbone-bonds to break by almost exclusively exciting electronic states associated with the latter. In EDD (the negative-ion counterpart to ECD), single-electron-captures of free, moderately low-energy (on the order of 20 eV) electrons (which in each anion results in the creation of a positive-radical or hole that exothermically recombines with one of the anion's negative charges) induce the peptides' inter-residue bonds to break by almost exclusively exciting electronic states associated with the latter. ECD and EDD require that the precursor ions be forced to mingle with a dense population of low-energy electrons. Since the reagent electrons and the multiply protonated precursor ions have opposite polarities and masses that differ by more than six orders of magnitude, the conditions for simultaneously confining them in the same volume of space cannot be satisfied in a purely electrostatic cell, and can only be minimally satisfied in an RF cell. To date, the only instrument in which it has been possible to achieve this condition to any practical degree has been the FT ICR mass spectrometer.
Since its advent in 1998, electron capture dissociation (ECD) has come to be regarded as a potentially powerful tool for elucidating protein structure. Numerous efforts to optimize ECD for protein analysis have been reported over the past decade. Less publicized has been a small number of recent attempts to overcome the limitation of ECD's original implementation, namely, the necessity for practical purposes of having to perform it on FT ICR instruments. Several researchers have independently succeeded in observing ECD in a linear ion trap, a three dimensional (3D) ion trap, and a digital 3D ion trap. (Baba et al., Anal. Chem. 2004, 76: 4263; Satake et al., Anal. Chem. 2007, 79: 8755; Silivra et al., J. Am. Soc. Mass Spectrom. 2005, 16: 22; Ding et al., Anal. Chem. 2006, 78: 1995.) In the first two of these demonstrations, magnetic fields were used for electron confinement, and in the last one, a digitally generated, rectangular-trapping, electric-field waveform was used for this purpose. In all three approaches, it was necessary to use a moderating gas (He) either to convert some of the electrons' translational energy into rotational energy about the magnetic field lines, to compensate for the unavoidable transfer of energy from the RF field to the electrons, or both. In the two 3D ion-trap demonstrations, ECD occurred in the analyzer itself, whereas in the linear ion-trap demonstration, it took place in a custom-designed cell. By virtue of being analyzer-independent, the linear multipole would seem to be a more promising platform than the 3D ion trap.
In any of the configurations described above, ions are vulnerable to losses in a mass spectrometer as they are transported from the ion source to the mass analyzer or between two mass analyzers. Electrostatic lenses, radio-frequency (RF) multipoles, and combinations of both are typically used to avoid or mitigate such losses. Unfortunately, the devices are complex, expensive, and frequently can be configured for only a limited range of applications. For example, conventional devices typically cannot be conveniently reconfigured to use a different dissociation process. In addition, in RF-field based devices, beam energy control is difficult because of beam interaction with the RF field. Beam losses are also high due to the dependence of beam propagation on the phase of the applied RF field. Thus, improved devices are needed to transport, trap, and dissociate electrically charged, gas-phase molecules (ions).